RALEIGH, NC (AP) -- Federal prosecutors have served North Carolina's environmental agency with 20 additional subpoenas seeking documents and ordering state employees to testify before a grand jury.

The move is part of a broadening criminal investigation triggered by a massive coal ash spill into the Dan River.

The subpoenas made public by the N.C. Department of Environment and Natural Resources on Wednesday are dated Feb. 11 and Tuesday. They came after two subpoenas The Associated Press previously reported were issued to the state agency and Duke Energy.

The new subpoenas seek documents related to the agency's regulation of Duke's coal ash dumps before the Feb. 2 spill, which coated the bottom of the river with toxic ash up to 70 miles downstream.

Duke has declined to share copies of subpoenas it received.

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